Making a 20" Gregorian.

[Billyboy78]

Hi, new guy to the forum. Why would you want to make a 20" Greg you ask?? Over the last 50yrs I've made an 8", 10", 16", and a 6" Berry refractor, started a 12" classic cassegrain nasmyth plus a purchased 8"Celestron XLT. The 16" was near the size that I wanted but at my age climbing a ladder with a heavy camera was impossible, besides it was too flimsy, all the time adjusting the counterweights and I hated the reverse image in that if the image drifts to the right commonsense tells you to move left, WRONG!, then you've lost it and you start again.

All this started me thinking a Gregorian with Nasmyth/ Coude' focus would be the go, squirting the image down the dec shaft and then the polar shaft of a GEM to a comfy chair where I can hang all sorts of equipment on the eyepiece and not upset everything. At this point in time to test the concept I've constructed a 18 point mirror cell, the 6 sided cage of 25mm square tubing to hold the tertiary diagonal in front of the primary, the dec shaft made of 3" steam pipe inside of 3" roller bearings in pillow blocks. The intersection of the dec/polar shafts will house the 2nd diagonal. The polar shaft is similar to the dec shaft with provision for the slow motion drive and goto encoders. When I described this idea on the CN forum I was politely told to make a CAD drawing and take a course in optical engineering.
They did however raise the question of the secondary and at the time I wasn't 100% sure what an ellipse was, the idea of 2 focal points made my head spin. So, now I've ordered an 8" blank to test whether I can grind it to the desired shape. To this end I made a turntable driven by 2 drill presses, an accessory belt and pulleys off my old Jeep, plus a KE tester. Did I mention I'm on a tight budget?

[FlashDrive]

:gday:Go for it Bill .... should be a great project....


:)

[billdan]

Welcome to IIS Bill, my knowledge of Gregorian telescopes is limited to what is described by Wikipedia. I guess making the secondary will be the most challenging being a concave ellipsoid mirror.

Once you are finished it should be an excellent telescope.
Wish you all the best

Cheers
Another Bill

[croweater]

Sounds good Bill :thumbsup: Post a few pics if you get time. Cheers, Richard.

[Billyboy78]

Thanks for the welcome guys.

An ellipsoid is midway between a sphere and a paraboloid and is easier to make than a convex mirror as in a cassegrain. The ellipsoid because of its shape has 2 focal points and when testing you put the light source on the near focal point and the KE on the other, which in my case is quite long, so its a direct measurement.

[Wavytone]

Billyboy,

I'd ask you think carefully about what you intend to use this for... I've used a 20" and 30" cassegrain before and apart from the planets there's photometry but not much else these were good for - the field of view was way too small, and the focal ratio too slow. I'm not sure if any amateurs do photometry anymore, either. It might be better left as a Newtonian. If you just want a planet-killer a 12" schiefspiegler offers more than sufficient aperture, adequate light grasp and is easier to make.

There is a reason why you will probably be disappointed by a gregorian - the monochromatic off-axis aberrations of the secondary mirror add to those of the primary - rather than to some extent correcting them as in the case of a classical cassegrain. Even if the spherical aberration is perfectly corrected on axis, the off-axis astigmatism and coma are much worse than in a classical cassegrain.

I would encourage you to make a classical cass instead, the secondary isn't impossible to make or figure with patience if you have the scope essentially finished to the point you can star test.

The Nasmyth idea is good, and on that size I expect you would be riding as a passenger on the mount and using electric slewing. The focal length of a 20" will be of the order of 6-7 metres which poses a significant challenge however ... I found non-tracking altaz mounts infuriating to use with a mak of focal length of just 2700mm, so at 6000-7000mm the problem just gets worse and you will have to make this track and probably GOTO just to be useable.

[Billyboy78]

Wow, let me clarify some points.

Primary :500mm F5 FL :2500mm.
Secondary : 200mm F4.5 FL : p 920mm x p' 3990mm
Standard german equatorial mount with 75mm shafts, 5mm wall thickness.
viewing angle : 27deg

Yes I've done ray tracing.....on my lounge room floor using pieces of cotton.

[Wavytone]

Back of fag packet calculation gives effective focal length 10,800 mm - making the scope almost f/22. Even worse than I guessed.

In a 2” eyepiece with an ID of 45mm the available field of view is a whopping 14 minutes of arc and a 40mm eyepiece will give 240X - if the seeing allows that much.

Good luck with that - you’re going to need it.

[Billyboy78]

Quite reasonable figures I thought.

[Stefan Buda]

There are very good reasons for not building large Gregorian telescopes - that's why nobody does, I guess.
One of the biggest reasons is that they are so much bigger than an equivalent aperture common Cassegrain, with all the resulting extra difficulties.

[Billyboy78]

Update: After a lot of good advice I'm thinking the old design was not very practical, so I've redesigned it as an 20" F3 and the secondary 8" f2.75. I've done full sized ray tracing and its shown to drop the overall length by 1.3 meters. I know the f3 will be a challenge but its nothing a bit of patience wont fix.

[Wavytone]

Bill how much mirror grinding experience do you actually have ? What you propose is not a small undertaking even for those with considerable experience and a grinding machine to do the hard work.

Two suggestions:

1. Make the primary with a long focal ratio - say f/7, and use the 8” as a flat secondary to reflect the beam back down the OTA. A secondary that big will result in an OTA around 2.5 metres which would be manageable as a pole truss.

From there use a second diagonal flat to bring the beam out to the side. Although it’s not a nasmyth, this would put the focus at a pretty reasonable height for most observers - and without needing ladder.

2. Make it a 20” f/5 conventional newtonian. A whole lot easier, and a lot more practical to observe with as well. Buy the elliptical flat secondary to match. Turn the 8” into an 8” newtonian, say f/4, and it would serve nicely as a super finderscope on one side.

Even when mounted as a dob you’ll need a ladder to use this... and ladders are very very dangerous in the dark.

[Billyboy78]

I got my inspiration from a gent called 'badbobastronomer' whose 20" greg wouldn't look out of place in a flintstones film set. He has a number of Utube videos describing the construction. The lesson here is that it works without much outlay. He's a man after my own heart and I figure if he can do it,so can I.

As for my experience, its all in post #1 1st paragraph, along with the grievances.

Cheers

[Wavytone]

Hmm... sorry pardon my skepticism, but someone on CN recently proposed making a large gregorian with rather fast mirrors similar to your numbers using sandpaper and a lot of elbow grease, and hoping to literally "eyeball" the figure to the desired shape with no more than a steel rule as measuring equipment...

Not you, by any chance ?

[Billyboy78]

LOL! No, rest assured its not me. 50yrs ago I thought of using an army searchlight mirror but I've moved on since then.

[sharpiel]

Wow. I hope that project got loads of encouragement. Just for fun reading...:rofl:

[Kunama]

That's the spirit, I would like to see this scope come to fruition Billy :thumbsup:

Though I dare say it would not be my first choice for a project :question:

[Billyboy78]

Update: Mirror blank ordered plus a truckload of 80 grit. A-grinding we will go ta-da.

Question.. what is the go with grit from a lapidary shop these days? I bought a kilo of 80 from the local shop, mainly because I go past it regularly, with the idea of using any rubbish to hog it out. They tell me they import all their grit from the USA so I'm thinking if there is a high turnover there will be a less chance of it being contaminated. They stock 46 down to 1200. It's a lot easier to buy local. I will use high grade Cerium Oxide.

[Wavytone]

Billy if in doubt you can grade the grit yourself using water. The procedure is described briefly in one of A.G. Ingalls ATM books, you should own these; if not BUY A SET on ebay.

Personally I wouldn't trust the grit from a source that wasn't a reputable one with a firm connection to ATM - lapidary supplies definitely do NOT qualify.

The test will be when you think you've ground out the pits of a previous grade, inspect with a loupe. If you see scratches, the grit ain't what its supposed to be.

Cerium oxide is for polishing - not grinding - and is an alternative for black-rouge. In my mirror-grinding days I tried both. Cerium oxide polished faster but produced a rougher surface. On small surfaces (secondaries and lenses) it was so aggressive it isn't a good idea as the figure will be terrible and uncontrollable. IMHO not suitable for mirrors under 30cm.

Rouge is slower to polish, but produced a much smoother surface and better figure and ultimately a far better result - with patience and a lot of elbow grease. If you have the patience I would use rouge - not cerium oxide.

I continue to have doubts about your experience... anyone who has ground a few mirrors before would know these basic things.

[dave brock]

Carl Zambuto uses cerium oxide. I guess he'll learn once he's made a few more mirrors?:lol:

[Wavytone]

As do most if not all making mirrors commercially where minimum time is important.

[Lognic04]

Whered you get that blank?:eyepop:

[Billyboy78]

Hi Logan
G James glass here in Brisbane. They have a water jet cutter. I've ordered it and paid but they are taking their time. I cancelled that 1st order for the 8" blank due to a personal problem. All go now. I pulled the fixed post turntable apart last thursday and finished it today, I felt bad that it was as rough as guts and somebody suggested all our work should be the best we can do.:lol: I also made up a number of concrete tools, I think casting plaster will be too soft for the machine,besides I wont need extra weight.

[Billyboy78]

Glass has arrived! Starting on #46 grit with a 3" steel ring. Taking a break to knock up a depth gauge using an old spirit level and a dial gauge. I'm not going to worry about a spherometer at this point, maybe later.

[Billyboy78]

3" ring is very slow. Ground a whole 1mm after an hour or so. 8mm to go. I've knocked up a 9" tile tool to get the machine to do a bit.

[sharpiel]

We definitely need progress pictures.

[Billyboy78]

Sorry, nothing much to see at the moment.

The #46 grit is only lasting about a minute on the tile tool, I'm forever recharging. I was looking for a diamond cutter to speed up the work and came across a diamond tool for shaping concrete that fits on my angle grinder. I gave it a 5 minute test and it looks like it may be the trick. The glass comes off in a dust so that may be a hazard, unless I take it outside and play water on it as I grind.

That depth gauge I made using a .001" dial gauge was useless, I couldn't get repeatable readings off it. I've now got a metric micrometer that I chopped up and using a lot better. The inflated figures on the old gauge were better to look at than this one.:lol:

[Billyboy78]

Yay. Just over 1 hour and I'm down 5mm in the centre, 3mm mid and zero on the edge. Playing the hose on it is keeping the dust down. I plan to stop at 8mm and take the last mm down with tiles on account its got a couple of gouges and I would hate to put in another.

[Billyboy78]

With 3 1/2 mm to go I stopped the heavy grinding to see how much of a hole in the centre I had. Did a bit with the 3" ring to tidy it up and then measured it with a 6" spherometer I knocked up. Turns out to be .005" variation over the face of the mirror (between .020 and .025) with 1" untouched around the edge so I'm pretty pleased with that.

[Billyboy78]

Hopefully a pic will appear courtesy of my whizz kid daughter.

What you see is the glass, spherometer , concrete grinder, turntable driven by a couple of drill presses, and in the background parts of the GEM.

[Lognic04]

Awesome! I am interested in how you made the grinding stand!

[Billyboy78]

Hi Logan,
The story really goes back 12 - 14 yrs when I discussed 'the perfect scope' with the guys on STAR Astronomy in New Jersey. I had finished the 16" newt and was starting on a 12" nasmyth cassegrain which I never finished, but kept the idea in my head. Over the last 12 months plans were drawn up and I started to collect bits that might be useful, including buying cheapo drill presses from the sunday markets and bits off my Jeep that I wrecked. The spindle comes from the bracket that held the mechanical fan, and when sat on its end has 4 bolts sticking up that are attached to 3 pieces of 12mm plywood, 1 x 24" D and 2 x 23 1/2"D. There are 3 x 4" swivel wheels under the circumference to stop any flexure it might encounter. The accessory belt from the jeep sits on the smaller discs and attaches to the grooved pulley which is attached to the drill press. I measured the height of the pulley and cut the height of the drill press to suit. The drill presses are set to their slowest speed along with a connecting belt between the two. Current speed is 16rpm but this can be upped at any time by selecting different configurations of the belts. The pieces of Unistrut you may see beside the turntable are for the fixed post.

Hope I didn't bore you.

[sharpiel]

Great pic. You could even rig the grinder on a mechanical arm to generate the required curve.

[Wavytone]

Ha... I was going say hang angle grinder at the end of a pendulum length = desired radius and let it swing !

Only snag is angle grinders don’t play nice with water :(

[AndrewJ]

What about using one of those flexible drill extensions in between.
Andrew

[Billyboy78]

Been there, about 12yrs ago making the 12". I attached the angle grinder to a bit of 3x2 and hung it from the apex of my roof via a ball joint.(There is a pic of it in the Brisbane Astronomical Soc newsletter) Went ok for the first pass but despite running water over it the glass began to splinter and I ruined it.

I've never had a problem with water either then or now. You don't turn the tap on full blast, just a trickle over the glass. The water gets flung off once it contacts the blade, nowhere near the electrics.

[Billyboy78]

I've seen one like that. The mirror was mounted vertical and the arm was horizontal and they made a pass, wound it in a bit and made a second pass.

When I made mine there was a lot of flexure, I needed 2 pieces of wood, one either side. If the arm was horizontal it would have to be braced to stop the arm sagging.

[sharpiel]

I think the grinder would need to be solidly held. They do tend to kick and have a life of their own when given "freedom".

Lots of food for thought anyway!

[Billyboy78]

Yes, on the next one that's what I would like to do. I've stopped heavy grinding 1.5mm short of the total sag for that reason. I liked the idea of that plywood one posted earlier, that is doable.

[Billyboy78]

Excuse me if I appear to skite, but I just gave the mirror a tickle with #220 then took her out into the sun to measure the FL, 1700mm, just shy of 1785 target. I've still got .5 mm to go on the sag and the sphere is not quite out to the edge. Gently gently we go with #80 grit.

[Billyboy78]

.25 mm to go.! I was still using the 9" tool but I want to grind the last bit with a bigger 15" tool but I couldn't make it until I had a spherical centre to use as a template. So, cement tool has been cast but I have to twiddle my thumbs waiting for it to cure before I can seal it with resin and attach the tiles. I have found resin to be cheaper than epoxy and it cures hard as a rock.

[Billyboy78]

Sag reached, beautiful sphere, #80 grit done. move onto #220 tomorrow. Fixed post grinding has to be the secret to a smooth figure.

[Lognic04]

Nice!

[Kunama]

Where are the pictures Billy :shrug:. Sounds like great progress... :thumbsup:

[Billyboy78]

#220 done. Did a FL test using my slitted led light source. Turns out we are a bit short, 1600mm instead of 1785mm target. F 3.2 but I'm not fussed, I don't want to mess up this figure

[Lognic04]

Awesome! Pics?

[Billyboy78]

Halfway through #400. Life keeps getting in the way. I'm limited to 15 minute wets due to the drill press motor getting too hot. I've got a desk fan blowing on it and I call a stop when I can't hold my hand on the casing. I'm using GW's technique of recharging on the go.
Confounded scratches, got a small one now and by the size of it it can only be the heavy concrete tool. Spent an hour yesterday bevelling everything down.

Pic's soon.

[Lognic04]

Nice!

[Billyboy78]

Started on #600 but after a short session I found a couple of pits I hadn't seen before. Went back to #400, then back to #220.:mad2: Now I'm back to #600 all looking good. I'm using an old film camera 50mm lens as a magnifier, I got no idea the power of it compared to a loupe but it is awesome.

[Billyboy78]

Feel free to say "I told you so" but I mistakenly presumed all the grits for optical and lapidary came out of the same factory. Wrong! The locally purchased grits from #40 - #400 worked great, it was the #600 and #1200 that were contaminated. Yes, I'm an idiot but I had to know. 40 yrs ago they said not to use them, but I thought things may have changed over time. Patiently waiting for some good stuff to arrive. They are only fine scratches that come out with 10 minutes of #600.

The delay has given me time to work on the mirror cell. I put the mirror into the cell for the first time and the 18 points worked great. Rather than expensive aluminium triangles, I used 1" x 1/8" metal strap welded into a T with the same dimensions as the PLOP design. The bit I worked on were the edge supports designed by Mike Lockwood using a wiffletree arrangement with tiny roller bearings contacting the mirror. I'm going to use the cell for any testing with the ke, ie sit the cell below the level of the ke so the mirror is resting on the 18 supports.

[Kunama]

Pictures Billy......... where are the pictures????????

[Lognic04]

Pics please!!!! :D
Dang, that is annoying :(

[Billyboy78]

Very much work in progress, and its not very pretty.

The mirror cell is attached to the frame by threaded rod used for coarse collimation, the secondary will have very fine tpi adjusting screws. The light path is directed down through those large bearings to an eyepiece. The FL of the mirror really determines the sequence of events in construction.

The other pic shows the grinding with a fixed post.

[Billyboy78]

What, no comments? Is it 'whose going to tell him its never going to work' or are you in awe of my brilliance :lol:? I'm happy with the progress so far.

[troypiggo]

Don't get frustrated, mate. This section of the forum is probably not as frequented as others. Please keep posting your progress and pics and the comments will come.

I've been following this thread out of interest, and can certainly say that I'm always impressed with the skill and patience of you ATM guys. Well done.

[multiweb]

+1 Always fascinated by anything to do with the black art of grinding glass.

[Peter Ward]

I’ve been lurking in the background also reading this post in sheer awe....true grit ! :)

[croweater]

Very impressed mate :thumbsup: Please keep some pics coming. I'd say plenty of people watching. Cheers, Richard

[Kunama]

That's better:thumbsup: impressive project :)

[Billyboy78]

So, here I was today twiddling my thumbs waiting for the fine grits and wondering how I overshot on the sag so much. It was down to below 3 and still had the fine grits to go I was starting to worry. It was then that I noticed the bar attached to the unistrut looked a bit askew, so I measured it and found it at least an inch too close to the centre of the mirror. As I was grinding it must have worked itself closer and closer without me noticing, thus grinding chunks out of the centre. After a bit of a think I decided if I was going to change to chordal strokes it would be advantageous to go the whole hog and aim for f3.5, or f4 even, as I have plenty of meat on the edge and I have read lots of bad things about f3 mirrors. After a couple of sessions with #80 the sag has started moving backwards .010mm. At least I'm keeping to form and making everything twice!

[Kunama]

If it's worth doing, it's worth doing twice! :D

[Billyboy78]

:rolleyes:

No sooner started on planing the edge down when I decided the tool needed re-tiling. I tipped the new tiles upside down and glued the shiny side onto the old tile, figuring #46 grit will soon take care of the rough underside. I enlisted the help of missus and son to glue the 240 tiles on before the resin cured.

Once the tool had cured I started on chordal strokes (as in, moved the tool to get 3 1/2" overhang) with #46 grit, measuring sag and sphere at each 15 minute interval. I changed to #80 and moved the tool to the recommended 2 1/2" after I had ground 1.5mm from the edge zone. Now I'm on #220, sag is 8.7mm, sphere is steady on .025", but 1 huge pit in the middle. I'm hoping by the time that is gone I should be able to move on to #400.

[Billyboy78]

Getting a bit hard to win here. That huge pit has turned into an airbubble! It showed itself after I planed that 1.5mm off the edge. Looks to be .5mm in diameter, not very big but there nonetheless. Have to go back to #80 again.

[Lognic04]

That's no good! :(
I wonder how many more restarts it will take? (I'm hoping for 0!)
At least with the machine you can sit back and relax, infact I am grinding 240 grit right now!

[Billyboy78]

So far so good. Airbubble gone, finished with #80, #220, midway through #500.
I had some suspect #400 and #600 and my guy sent me some good #500, so to save anymore messing about I'll spend extra time on the #500.
Don't you just love the silky smooth feel of the polished glass?

[Lognic04]

Nice!
Yeah it is great!!!

[Billyboy78]

I'm on #1200 (5um) and keep getting persistent scratches, not very deep, like they come out after half hour of grinding. I'm thinking it might be the old grit sticking to (embedding) into the polyester resin that I'm using to seal/stick the tiles on (yes the resin is above the bottom of the tile) and despite scrubbing out the channels with a toothbrush they are dropping off and giving me grief. I'm going to give the tool another coat of resin to try and bury the monsters, but this means no more grinding for a couple of days while it cures.

There is an up side. The rest of the mirror has taken on a brilliant shine.

[Billyboy78]

Yay, fine grinding done. Make up the lap tomoz.

[Lognic04]

Awesome!

[Billyboy78]

Lap #1 was a practice run.(koff koff). Turned out hard as a rock. Trip to the freezer, start again, this time with the full 100 mils of castor oil. I used GW's method of pouring a puddle on the mirror and settling the tool in the middle. Now it is channelled, scored and cold pressing. Let the fun begin.

In the meantime I hogged out the 8" secondary and made a 6" tool out of concrete ready for the tiles. Now that I know the 3.4fl of the primary I'll aim for f3 for the secondary.

[Lognic04]

Wow 8 inch secondary!!!

[Billyboy78]

yeah, but its only 50 square inches, primary is 314 square inches, gives me 264 left.

[Lognic04]

Cool, How'd you hog out the secondary so fast?

[Billyboy78]

Go back to post #30 and there is a pic of the concrete grinding tool on the angle grinder. Took about 10 minutes.

[Lognic04]

Ah angle grinder. I should have done that for my 6 inch! Doh!

[sharpiel]

It's just cat fishing without pics :D

[Lognic04]

Can you share how you used the angle grinder to hog it out?

[Billyboy78]

Lap #3, (lap #2 still too hard) followed GW's instructions to the letter in preparing the lap. Lots of cerox, cold pressed for 2 hours, now I've locked it up and no amount of pushing/pulling (1 metre long lever) will move it. In my haste I was going to seal the back of the plaster tool for another day, so now if I toss it in the bath for a day I will ruin the plaster.:mad2: Do not want to make another tool!

Remedy will be to seal the tool in situ with resin or enamel paint, then dunk it.

[Lognic04]

sounds like you didnt use foil over the mirror?

[Billyboy78]

I just laid the glass on a rag and played a trickle of water onto it, then gently dug the edge of the cutter into the glass, working the middle of the glass more than the edge. In fact I didn't shave the edge at all, leaving about 2mm gap.

Because the cutter is 100mm diameter I couldn't lay it flat as I could with the 20" thus leaving a number of shallow score marks that will come out with #80 grit.

[Billyboy78]

You use foil for pouring the pitch but not for cold pressing.

https://www.youtube.com/watch?v=s4mXM7HyVJk&t=130s

[Lognic04]

Thanks!

[Billyboy78]

No more Mr niceguy! Laid the mirror/ tool on the patio floor, tied the mirror to a post, then got my 1/2 ton block and tackle around the tool and pulled! YAY, moved it. No apparent damage. Shelved for the moment though due to interest in the secondary, in that I'm on coarse grinding and I've got #46 grit everywhere.

[Lognic04]

46 Grit on an 8 inch?!!

[Billyboy78]

I like living on the edge!

[sharpiel]

I wish you were videoing or at least taking pics of all this. Seems a shame...

[Billyboy78]

I'm not too pleased with the resin lap that I have. The surface is quite sticky and if I start using it, it is going to lock up again, so I've ordered some Acculap and I'll toss what I've got. The plaster tool is junk also, I'll make a concrete one like the grinding tool.

[Billyboy78]

It would seem the fixed post doesn't like fast mirrors. I think Logan found this out also. Yesterday afternoon I worked for several hours on the 8" with the 6" tool to deepen the centre with #220 using a variety of overhang settings, but without success. Sag went down to 3.6 mm but no further. Then I resorted to manual grinding with #80 using very short strokes COC, small W's and anticlockwise circular motion against the flow of the turntable. So, after a couple of hours we are down to 4.12 mm with a target of 4.5 mm. Pretty pleased with that. Downside is I'll have to finish of the mirror manually including polishing.

[Billyboy78]

Manual grinding was not in this job description! Did I mention I was 74? After 15 minutes of stroking and my arms are dropping off. Should I shelve it? NOT ON YOUR NELLY MATE!!! Things are getting interesting.

Sag reached, Sharpie good, on #500.

[croweater]

74 wow Bill. Keep going mate. Watching with interest (and admiration). Photos if you get time. Cheers, Richard :thumbsup:

[Billyboy78]

Fine grinding done. Waiting on the Acculap. Making up a variety of different size tools for the pitch.

[Lognic04]

Ive found acculap works great!
I know it is necessary with a 20, but i wonder if subdiameter tools are useful on a 6?

[Billyboy78]

It would be good to target a particular zone without ruining other zones.

[Billyboy78]

Acculap has arrived in just 9 days from USA. Excellent service.

[Lognic04]

How much did you buy?
Acculap is great!

[Billyboy78]

2 KG / 6 packs. Intermediate

[Billyboy78]

Really all you have to do is look at Gordon Waite's tutorials on making a 20" f 4.5 and you see mine. I'm following it to the letter.

When I see the high standard of workmanship on this site makes me want to not show off my stuff-ups. Besides I need to tie down my daughter to load the pics because I am a total noob on that sort of thing.;)

[Billyboy78]

Made up the new polishing lap using Acculap. Came to making channels and started using a saw. Too slow! Broke out the angle grinder with a 1 mm blade, had them done in a few minutes. Interesting phenomenon, the pitch came off in the finest gossamer spiders web. Would have been good for Halloween. My patio was covered in it.

[Billyboy78]

About 1/2 hour polishing. Took a look at the surface to see what it looked like using a ronchi made out of a fine piece of wire. Straight as a die, very pleased.

[Lognic04]

Pics! Pics! Pics!

[Billyboy78]

(20") My yahooing was a little premature. I had a better look today with a ke tester and it turns out I have a central hill. No biggy, I'm not quite finished polishing so I'll try moving the tool in a smidgin to see if it will fix itself.

I bought a laser collimator yesterday to use later but good for testing the haze on the mirror. I would have said the mirror done but laser says no.

It's a bit ironic that I was following Gordon Waite's tutorial and I note he also had a central hill prior to parabolizing.:doh:

[Lognic04]

I have actually been using a laser collimator too!

[Billyboy78]

I need to upgrade my turntable. I no sooner get the lap warmed up and working properly when the motor gets too hot and I've got to quit the session. The session only lasts about 10 minutes when it really needs an hour or more. If you recall I've got 2 el cheapo drill presses joined together but they are not designed for continuous running, I need a 1/2 horsepower.

Even though I'm nearly finished polishing I've still got the secondary to polish.

[Billyboy78]

Picked up a motor at the markets this morning, now I need a pulley.

I've been working on the UTA, fashioning the 6 x 25mm ally tubes up to the upper cage. I didn't want to buy 6 x 3m tubes because of the expense, I opted for 3 tubes and cut them in half, which left me with a longer than usual upper cage. I think I said before I've got coarse adjustment on the mirror cell and the secondary has got fine adjustment on the outer edge of the spider, so the entire spider, secondary backing and secondary all move as one. Apparently the mirror spacing is quite critical so I want to try and get it right the first time. (Yes, pics!)

[Billyboy78]

(20") New motor up and running, did a solid 1/2 hr polishing, then did a comprehensive test. Haze test is good, Ronchi test is good, KE slight raised hump in the centre and a little zoney. GW says to smooth it with a 12" lap, so that's what I shall do.

[Lognic04]

Just curious, is GW's advice directly or some sort of forum?

[Billyboy78]

Gordon Waite has a whole bunch of tutorials on Utube. His company, Gordon Waite Research, manufactures a Renegade 20" Dobsonian, and he lives in New Jersey. He personally gave me advice about 12 yrs ago when I was making my 16".

[Billyboy78]

Had a bit of a rest from pushing glass and did a bit of work on the uta and made up the secondary holder. Its just a 8" circle of ply held flat by 3 lengths of 25 x 1mm aluminium spider. The spider is attached to 3 dovetail slides and there is a length of 3/16" threaded rod through the middle for fine adjustment.

In the meantime I started reading ATM book 1 for the 3rd time. You know how I said the mirror was done, well after reading ATM for a bit, maybe its not so done after all. So, I did 2 x 1 hour sessions at 10rpm and no weight and the surface improved no end. I may do another hour or two then tackle the slight hump in the middle.

[Billyboy78]

Did a bit of work on the central hill with a 4" tool. It's very time consuming when you polish for 5 minutes, then let the mirror cool down for 30 minutes, then test for a bit, then cold press for another 30 minutes, then the rellies come and want to know what I'm doing. At the end of the day the hill is diminishing, ready for a new 12" lap to try and smooth the surface.

[Billyboy78]

Dumb! Did the aforesaid 3 hours smoothing on the 20" without rotating the mirror on the turntable. Even when testing I would pick it up, place it on the test frame, plonk it back on the turntable and carry on never assuming for a minute I was doing a bad thing, until it was too late. Now I have polished in some astigmatism! The book blames the turntable. I'll have to strengthen it a bit and put in a 4th jockey wheel. Probably the 380mm x 40mm concrete tool didn't help. It's already 3 layers of 12mm marine ply so you would think it was ok.

[Lognic04]

Any news?

[Billyboy78]

Hi Logan.
Nah, but I do think about it occasionally though. Its been too hot and humid to play in my tin shed. But thanks for asking, I'll keep you informed on any progress.

Bill

[Lognic04]

hope you brought the pitch lap inside! :lol:

[Billyboy78]

Back at it. Whipped up a new 6" lap for the 8" secondary and spent 2 hours polishing. KE looked good.

[Lognic04]

Nice! Ronchi pics please!! :)

[Billyboy78]

I don't have much faith in Ronchigrams. When I was measuring the 20" the ronchi looked clean as a whistle but the ke showed a distinct raised centre.

Polishing is done on the secondary, now to figure an ellipse.

[Billyboy78]

Ellipse done. I used a 4" star lap and worked on the centre zone for about 10 minutes. Knocked up a bench tester to test the 2 focal points and after a bit of messing around got it to work. I had always thought that the secondary radius of curvature and the shorter P focal point were one and the same, and constructed the upper cage to suit. Turns out to be about 100mm shorter, didn't like the cage anyway.

Working to clean up the primary before I can star test.

[Billyboy78]

Pressure is on. My daughter has just bought a house with a granny flat and wants us to move in with them. This means I have 6 weeks to get the scope up and running 'cos I don't want to have to dismantle the turntable and test equipment and reassemble it in the new house.

I'm using a 12" star lap atm but there is not much movement yet.

[Lognic04]

uh oh!
At least it's F/5 - you should easily be done in that time! :)

[Billyboy78]

I wish..The 20" primary is F/3.39 and the 8" secondary is F/2.46.

Still battling the astigmatism I polished in. KE says it is around 1mm deep, so nearly there ( ie, 1mm variance in fl). The edge correction has got to be zero so I want to get it as good as possible. Centre correction btw is 18.76mm:eyepop:

[Billyboy78]

Scary stuff this. Figure and smoothness getting better all the time. I'm at my target of 18mm correction but the 50% zones need smoothing. I'm using a 6" star lap to dig out the centre and a 12" star lap to smooth it out.

A metric digital depth gauge should arrive on Wednesday to replace the jerky imperial dial gauge I planned on using. Thousandths of an inch make my head spin.:lol: I plan on making a graph comparing calculated to actual zone readings.

[Billyboy78]

I thought I could clear up the last of the astigmatism during the parabola session but it is not to be. I got the 15" lap out of storage and did a 1 hour stint with the plan to go back to a sphere, but I may not have to. I had a white and a black tadpole chasing each other across the middle of the mirror but this has come good. Fingers crossed.

[Billyboy78]

Nah...gone back to #500grit with the old tile tool. Did 20 minutes now on to #1200.

[Billyboy78]

Update....

Simply, it appears my super dooper turntable is not up to the job and I need to make a new stronger one, even though its going to be used hopefully for a very short time. This however is going to cost + time, both I don't have.

I have come up with a solution though for a cheap strong true turntable. Use a rear differential out of a car, mounted vertical with a mortar filled rim on top.

[Billyboy78]

Update...
House moving has caught up with me. I've dismantled the workshop and packed everything and tossed out a lot of valuable junk. Once I move then I have to build another workshop so it might be a while before I report again.

[Billyboy78]

What can I say. The new house is totally unsuitable for a large telescope to be housed with overhanging trees all around and a small backyard dominated by a 10m x 4m pool. Therefore I must GIVEAWAY the scope to any interested person in the Brisbane area.

Read this entire thread to see the fun I've had in building it and whats left to complete it. I can deliver down to the border/T'mba/B'berg etc. for a small charge.

Call me on 0421 159 330.

[Wavytone]

You mean to say you started making this without understanding the implications of mounting and housing such a beast ? :rofl::rofl::rofl: